about the rank insignias, OTL, British Empire / Commonwealth countries are pretty much in the minority in terms of having distinct insignias for their various uniform branches. Most countries actually have a single rank insignias system used by all.
I know. Even my country has only very basic differences for the appearance of the rank insignias in the ground forces and the air force, and the terminology for the ranks is the exact same in both the army and the air force. There's no unusual rank name in the air force at all, they're all the same names as the ranks in the ground forces. Interestingly enough, we've not had the rank of
práporčík and its close derivatives (sort of like
"ensign", just lower in rank) since about 2015 or thereabouts. Some of the lower NCO ranks were instead moved up and replaced it.
If you want to have distinct insignias for all 3 branches, maybe design a basic template and modify it for all 3 branches (ex: low officers use 1-3 straight horizontal stripes for army, 1-3 wavy stripes for navy & 1-3 zigzag stripes for air force)
In the case of my fictional Kymria, I've taken most of the terminological inspiration from British, Austro-Hungarian and some other central European terminological traditions, and I've tried to make it as consistent as possible. The rank names themselves are conventional, and differ very little from the sort of ranks you'd find in the OTL world during the last ca 200 years.
Nota bene, I've done this attention to consistency in multiple languages (the Welsh-sounding one is the main one, but there are also minority languages heard in the forces, and even a Kymria equivalent to
"Army Slavic").
On the visual side of things, I've largelly gone for an original approach, no stars, and only the three topmost ranks of the main branches have a royal crown. The insignia is instead rooted in subtle but logical colour-coding and in a system of symbolic insignia largelly unique to Kymria and its national vexilology/heraldry.
There's also not going to be a mess of unique names for nearly every service branch's ranks, unlike in both real British tradition or Austro-Hungarian tradition. They do have a system to specify which soldier comes from which branch, but unless there's a good reason to mention it, it's not added to the rank name as standard. Thus, everyone just goes by the standard army, navy and air force rank they have. For example, you can have two sergeants of the army and air force meeting, and neither of them will have a convoluted long name for their rank. They'll just be sergeants.